1. Repeated biweekly intraperitoneal injections of mice with lard solutions of methylcholanthrene and phenanthrene for 18 weeks did not affect the hepatic stores of vitamin A. Methylcholanthrene induced rapidly growing sarcomas and the production of ascites. Tumors of the liver were not observed. Neither phenanthrene nor lard produced tumors or ascites. 2. The influence of large doses of intraperitoneally administered phenanthrene, 3,4-benzpyrene, and methylcholanthrene on both hepatic vitamin A storage and growth in young rats was also studied. Methylcholanthrene produced a marked reduction in hepatic vitamin A, and 3,4-benzpyrene had a comparable but somewhat less pronounced effect. Phenanthrene exhibited the least potency, only reducing the hepatic stores about 20 per cent of the normal. With methylcholanthrene and benzpyrene, the depletion of hepatic vitamin A was much greater than the inhibition of growth. In the case of phenanthrene, there was some correlation between the decrease in hepatic vitamin A and retardation of growth. Methylcholanthrene and 3,4-benzpyrene produced ascites in some of the rats. 3. The intraperitoneal administration of large amounts of phenanthrene, 3,4-benzpyrene, methylcholanthrene, and 1,2,5,6-dibenzanthracene in the presence of massive doses of vitamin A administered orally neither appreciably affected the vitamin A stores of the liver of mice nor interfered with the assimilation of the vitamin. Ascites induced by methylcholanthrene was less pronounced in the presence of added vitamin A. Although 3,4-benzpyrene produced ascites in several mice, this reaction did not occur in mice treated with dibenzanthracene or phenanthrene. 4-Possible explanations for the differences in response of the mouse and rat to these hydrocarbons are discussed. The use of more than one species of animal, at least for certain phases of experimental cancer research, would seem to be indicated. © 1942, American Association for Cancer Research. All rights reserved.